A New Detection Technique for RH and EMI Field Sensors for Measuring Both Peak and RMS Value

Abstract

Thermosensitive and diode based instruments This paper describes the use of a signal multiplier as a detector in order to perform both peak and RMS measurements in an electric/magnetic isotropic field sensor. Since thermosensitive instruments show a high time constant they can measure the RMS value only. The diode based instruments, using the "square law" region of the diode, measure the RMS value of the field but cannot measure the peak value; if used in the linear region they can either measure the RMS value by squaring x, y, z measurements or detect the peak value by using three synchronized peak holders. The proposed detection technique uses three signal multipliers as load of three orthogonal dipoles or loops that perform the square of the signals without loosing the mutual phase information. Then the three signals are added to obtain the square of the instantaneous value of the electric or magnetic field. This signal applied to an integrator (RC circuit) gives by definition the square of the RMS value for all types of polarization. From the signal sum it is also possible to detect the peak value. An experimental implementation of an electric field sensor using this technique and some results of various measurement situations are herein reported. A very good dynamic range has been achieved. The existing suitable instruments [4] , measuring electromagnetic fields, can be divided in two families: thermosensitive instruments, diode based instruments. The first family uses a thermometric or bolometric device as load of a dipole or a loop; in other types the electromagnetic wave impinges directly on the thermosensitive device. In both cases a thermal conversion of electromagnetic energy is done and the true RMS value is measured. But this transfer function is intrinsically slow and does not permit to measure the peak value. The time constant of these systems is of some seconds. By working with three orthogonal detection systems, an isotropic sensor is obtained and the measure becomes independent from the polarization because the sum of the RMS values of three orthogonal components of an arbitrary polarized wave is, in fact, the RMS value of the wave. Consequently this result gives a DC signal related to the following function: 2 Erms 2 2 2 Exrms + Eyrms + Ezrms ( 1 ) Introduction 2 2 Hrms = Hxrms + Hyrms + 2 Hzrms (i ■) The measurement of electromagnetic radiation is a very complex discipline [l,2] and it is difficult to meet all the requirements of different users . However many electromagnetic field sensors have been developed to measure relative strong fields in EMI and Radiation Hazard environment [3,4]. In EMI activities these sensors are employed to perform . radiated susceptibility tests at equipment or system level (e.g. RS03 MIL-STD-461) or to characterize sites like a transmitter tower, a high voltage power line, etc.. In this way, the scalar value of the field for all waveforms (CW, modulated signals) and all polarizations must be known. Both true RMS and peak value are necessary to characterize an electromagnetic field with respect to the nonlinear effects on the electronic devices and to the digital systems. On the other hand the electromagnetic field must be quantified with respect to the peak and RMS value for Radiation Hazard purposes, because their ratio is an important parameter to define the risk factor: the nonthermal effects (e.g. the microwave hearing) must be considered as well as the thermal ones [5]. The measurement must be done for every type of polarization, propagation direction and waveform. This kind of sensor can measure RMS values only. The diode based instruments use a Low Barrier Schottky Diode [6] as load of dipoles or loops mounted in an orthogonal scheme. The diode can operate in the "square law" region and/or in the linear region of its I vs V characteristic. Assuming to operate in the "square law" region (this condition is satisfied if the voltage across the junction is less than 100 mV), a DC signal proportional to the square of the true RMS value of the field appears on the diode [7]. The orthogonal geometry permits again to measure the true RMS value of an electric/magnetic field with arbitrary waveform, polarization and propagation direction. Unfortunately the "square law" region exhibits a high differential resistance which does not permit to have a transient response faster than the time constant, strictly connected to the lower frequency limit. It is possible to demonstrate [7] the equivalence shown in Fig. 1 (e.g. electric dipole). Einc is the component of the incident field parallel to the dipole, heff is the effective length of the dipole, Cant is the dipole equivalent capacitance, Cj and Rj are respectively the junction capacitance and the differential resistance of the diode and a is a constant depending upon the diode characteristics. In this way it is not possible to measure both peak and RMS values. CH2294-7/86/000-0019 $ 0 1 .00 © 1 98 6 IEEE 19